'Intolerable'

White House spokesman Josh Earnest has called on the parties to work towards a "sustainable ceasefire".

"Hamas' decision to resume rocket fire will not only put the people of Israel and of Gaza at greater risk, it will do nothing to meet the expectations of the Palestinian people," Mr Earnest said.

UN Secretary General Ban Ki-moon also condemned the resumption of violence in the region, calling the deaths of civilians "intolerable".

The violence resumed after Israeli and Palestinian representatives failed to agree a long-term truce at indirect talks in the Egyptian capital, Cairo.

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Palestinians try to salvage books in a badly-hit mosque amid ongoing air strikes in Gaza City on Saturday

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Israeli families were forced to flee their homes on Friday as a rocket hit the southern city of Sderot

Hamas says the major sticking point in negotiations is Israel's refusal to meet its demand of lifting the blockade on Gaza.

It is also seeking the release of about 100 people released in 2011 in a prisoner exchange deal but re-arrested in the past six weeks.

Hamas rejected Israel's call for the demilitarisation of Gaza.

However, the group said the Palestinian factions were willing to continue the talks despite the fresh violence, and a Palestinian delegation met Egyptian mediators in Cairo late on Friday.

But Israeli officials, who have pulled out of talks, said they would not "negotiate under fire".

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Israeli forces clashed with Palestinian protesters at the Bek Forik checkpoint in the West Bank on Friday

Senior Palestinian official Mustafa Barghouti told the BBC on Friday that rocket fire into Israel had come from other Palestinian factions, and not from Hamas.

The Israel Defence Forces (IDF) said it resumed its offensive with aircraft, tanks and gunboats after militants began firing missiles from Gaza shortly before Friday's ceasefire ended at 08:00 local time (05:00 GMT).

More than 70 rockets had been fired from Gaza into Israel since the truce expired, it said on Twitter.

Israel launched Operation Protective Edge on 8 July with the stated aim of halting rocket fire from militants in Gaza and destroying the network of tunnels it said were used by militants to launch attacks inside Israel.